This invention relates to a noise generator for use in telephones and other communication devices wherein it is desired to avoid complete silence during a communication.
As used herein, “telephone” is a generic term for a communication device that utilizes, directly or indirectly, a dial tone from a licensed service provider. As such, “telephone” includes desk telephones, cordless telephones, speaker phones (see FIG. 1), hands free kits (see FIG. 2), and cellular telephones (see FIG. 3), among others. For the sake of simplicity, the invention is described in the context of telephones but has utility in any communication device that silences a channel temporarily.
Anyone who has used a speaker phone, for example, is well aware of the cut off speech and the silent periods during a conversation caused by echo canceling circuitry within the speaker phone. Such phones generally operate in what is known as half-duplex mode, which means that only one person can speak at a time. While such silent periods assure that sound from the speaker phone is not coupled directly into the microphone within the speaker phone, the quality of the call is poor.
Telephones of the prior art often impose a silence in an attempt to eliminate acoustic and electronic echoes. When speech is gated off by a center clipper, attenuated by a residual echo suppresser, or canceled by a noise cancellation system, the resulting output is unnaturally quiet. The silence has been interpreted by consumers as a broken connection and a party to a call might mistakenly hang up. This problem has been solved by providing so-called “comfort noise” in which a low level noise signal is applied to a line rather than silence. U.S. Pat. No. 6,122,611 (Su et al.) describes a system that not only adds noise during periods of silence but also adds a little noise during conversation to avoid changes in the apparent loudness of the speech.
While one might think that all noise is the same, such is not the case. An automobile produces quite a different background noise from an office or a living room full of people. Adding “white” (spectrally flat random) noise produces yet another background sound. U.S. Pat. No. 5,657,422 (Janiszewski et al.) discloses filtering the noise in a low pass filter to make it sound more natural. While better than white noise, it remains a problem to provide a comfort noise that resembles the actual noise in each individual telephone call.
In view of the foregoing, it is therefore an object of the invention to provide an improved generator of comfort noise.
Another object of the invention is to provide comfort noise that more closely matches the spectral content of actual noise during a call.
A further object of the invention is to provide a comfort noise that matches actual background noise as closely as possible by shaping white noise using a quadrature mirror filter bank.